"Due to an unacceptable side effect profile and uncertain [anti-nausea] effects, intravenous THC administered at the end of surgery prior to emergence from anesthesia cannot be recommended," said Swiss researchers led by Dr. Lorenz Theiler of the University of Bern.

The negative findings were so clear-cut the researchers halted their study early.

The study included patients undergoing gynecological or breast surgery who were randomly chosen to receive either intravenous THC or a placebo at the end of surgery. The study was supposed to include about 300 patients, but was stopped after 40.

Among those given the marijuana extract, the compound was found to reduce nausea and vomiting risk only 12 percent of the time. The study authors said this compares poorly with standard anti-nausea medications, which have an effectiveness rate of 25 percent.

Patients given THC were found to take longer to recover from surgical anesthesia and felt woozier once they did. THC patients also tended to spend more time in the recovery ward after surgery.

Psychological side effects were "unpredictable in both quantity and quality" in the THC group, the researchers said.

On the plus side, those given THC tended to require less pain medication, the investigators noted.

Nausea is a common concern following surgery. Because standard drugs don't fully eliminate the risk, researchers have set about looking for potential alternatives.

The researchers behind the new study focused on THC because of evidence it eases nausea related to chemotherapy, they said.

The findings were published recently in the journal Anesthesia & Analgesia.